By way of example, the signal source which is to be protected may constitute an electrical signal amplifier. A short circuit in a load of the amplifier can cause the latter to supply current beyond its intended capacity and thereby to create an overload condition. In order to prevent the overload condition from damaging the amplifier, it is necessary to sense the onset of such a condition and to immediately terminate it, e.g. by turning off the amplifier, or by taking other appropriate action for preventing or limiting damage to the amplifier.
Prior art protection devices which are intended to operate in such situations typically sense the overload condition and turn off the amplifier in response. A common failing of this type of device is that it permanently turns off the amplifier. Should the overload condition be temporary or self-curing, rendering permanent turn-off unnecessary, the amplifier nevertheless remains turned off and must be reactivated by some predetermined procedure or program, or by the intervention of an operator.
Another disadvantage of many such prior art protection devices is that they utilize a sensing device having a slow response time, such as a sensing element heated by an overcurrent. Such prior art devices have limited effectiveness in that they respond only to overcurrent conditions of suffiently long duration at a predetermined threshold amplitude to trigger the sensing device. Thus, these slow-response devices sometimes allow damage to the amplifier to occur during the time interval which precedes the triggering of the device. Conversely, fast-acting protection devices found in the prior art are sometimes prone to false triggering by spurious signals that may be present in an electrically noisy environment.
A further disadvantage of some prior art devices of this type is that they respond only to overcurrents of a fixed magnitude. Thus, they are either not adjustable by the user, or they can be adjusted only with difficulty.
Accordingly, it is an object of the invention to provide a new and improved electrical protection circuit.
It is another object of the present invention to provide a new and improved electrical protection circuit for turning off an electrical signal source in the event of a current overload, followed by automatic turn-on of the signal source after a predetermined time interval has elapsed.
It is a further object of the present invention to provide a new and improved electrical protection circuit for turning off an electrical signal source in the event of a current overload, followed by automatic turn-on of the signal source after a predetermined time interval of selectable duration.
It is still another object of the present invention to provide a new and improved electrical protection circuit which is selectively adjustable to respond to overload conditions of different magnitudes.
It is still a further object of the present invention to provide a new and improved electrical protection circuit which responds rapidly to an overload condition.
It is yet a further object of the present invention to provide a new and improved electrical protection circuit which is relatively immune to false triggering by noise transients.